1. Field of the Invention
The invention relates generally to data processing systems. More specifically, it relates to data recording systems which must reliably store large amounts of data while operating in potentially severe environments.
2. Description of Related Art
It is well known that there exists multiple types of storage media on which to store data in a computer system. Mass storage devices such as magnetic tapes have been used for over 35 years to record and storage data. Magnetic tape is a sequential storage medium consisting of a long and flexible ribbon coated with magnetic material. Tapes used in these storage systems vary in width from 1/2" to 1/4", with 1/2" as a commonly used width. The amount of data that can be stored on a magnetic tape depends on its length and recording density. Densities are available in 800, 1,600, or 6,250 bits per inch (BPI) for standard computer tapes. Storage capacities can range from 40 million bytes (MB) to 180 MB and beyond. Such standard magnetic tapes offer a relatively low-cost and proven media for archival storage. However, magnetic tapes are bulky and can be easily contaminated by improper handling or by being exposed to environmental dangers. Because magnetic tapes are sequential storage devices, the speed with which data can be located and retrieved is relatively slow. Additionally, the life span of the magnetic tape is limited. Over time, important information achieved on magnetic tapes may no longer be accessible because of the deterioration of the tape itself. Thus, using magnetic tapes as a preferred mass storage media is not ideal for many situations.
Magnetic disk memory replaced magnetic tape drives in many computer systems because they offered greater speeds and faster access to data due to randomly addressable access to selected tracks on the magnetic disk. The Winchester disk technology, developed by IBM Corporation, is widely used in the primary mass storage markets. Magnetic disk memory systems are known generally and are available in a variety of sizes, operational characteristics, and storage capacities. Examples of widely used magnetic disk memory systems are the Elite 5.24 inch STA41201J drive available from Seagate Technology Corporation, and the M2261 available from Fujitsu. Removable magnetic disk memories are also available, but the storage capacity of these disks is typically limited to around 100 MB. The commonly used magnetic disk memory systems are a popular and proven technology used by all categories of computer systems. They are sealed, low-maintenance products with a high mean-time between failure (MTBF). However, magnetic disk memory systems are vulnerable to shock, vibration, excessive temperatures, and electrical surges, as well as microscopic dust particle surface contamination.
In recent years optical storage devices have emerged as a promising storage technology because of their large storage capacities and compact format. There are three types of optical disk memory systems: read-only optical disks, which include CD-ROM; Write-Once-Read-Many (WORM) devices; and erasable optical disks, also known as Write-Many-Read-Always (WMRA) devices. Erasable optical disk memory systems come closest to functioning like magnetic disk memory systems because data can be stored, retrieved and written over with ease, and are made in a variety of sizes and capacities. Typical commercial WMRA systems offer between 256 MB and 650 MB per side of storage capacity on a 5.25" disk and support an interface to a computer via a generic I/O bus such as the Small Computer System Interface (SCSI), or the Intelligent Peripheral Interface (IPI), which is an ANSI standard interface. Optical disk memory systems available commercially include the RO-5030EII made by Richo Corp., the SMO-S501 made by Sony Corp. of America, the HP C1716A made by the Hewlett-Packard Co., and the OD112-1 made by Hitachi America. Ltd. Functionally, WNRA disks are rugged and offer massive storage capacities in removable cartridges. They can store multimedia categories of information including images, video, and digital audio that would not be cost effective on magnetic media.
Optical disks are less vulnerable to physical damage than magnetic disks or magnetic tapes. The optical disk media has a long archival life and can store massive amounts of data in a compact space, yet the data can be quickly retrieved. The cost per MB of storage is lower than for magnetic disks. However, the successful operation of an optical disk memory system is more prone to environmental challenges to data integrity than magnetic disk memory systems. Shock, vibration and other physical stresses can cause errors when reading data from or writing data to the optical disk. One solution to the problem of reading an optical disk when the system is subject to vibration and shock is disclosed in U.S. Pat. No. 4,796,247 to Vogelsang. Vogelsang teaches only that a look-ahead read/write memory can be used to compensate for tracking jumps due to vibration and shock during a read operation, but does not address the problems of writing an optical disk in an adverse environment. The table below shows the comparable bounds of safe, reliable operation of the optical disk memory, magnetic disk memory, and semiconductor memory (also known as solid state disk memory) systems.
______________________________________ Environmental Characteristics of Storage Media Semiconductor Magnetic Optical Characteristic Memory Disk Memory Disk Memory ______________________________________ Low Temperature -55.degree. C. -20.degree. C. 0.degree. C. High 125.degree. C. 90.degree. C. 50.degree. C. Temperature 100% Humidity .about.1000 Hours 100 Hours 1 Hour Shock .about.1000 G's 150 G's 30 G's Vibration .about.20 G's 4 G's 1 G ______________________________________
There are significant environmental limitations on the successful disk memory system will not operate well in environments that are below freezing, consist of high humidity, or contain shocks and vibrations. The magnetic disk memory system performs better than the optical disk memory system in all factors measured. The semiconductor memory system is the most rugged and easily surpasses both the optical disk memory system and the magnetic disk memory system in severe environmental conditions.
The criteria used in selecting a mass storage media for a computer system includes many factors, including cost, storage capacity, access time, transfer rates, reliability, power consumption, physical size, ease of system integration, service and maintenance, and data security. What may be and ideal definition of a computer system in a commercial environment may be unacceptable in a severe environment. In severe environments the computer system may be subject to physical stresses, including temperature extremes, shock, vibration and high humidity. The failure of the mass storage media under these conditions can cause the computer system to be unusable, resulting in data loss or corruption and a potential negative impact on human safety.
The storage capacity of optical disks is very competitive with other forms of mass storage, but the performance of the optical disk memory system in a severe environment is rather poor. The magnetic disk drive is sturdier, but it does not have the storage capacity of the optical disk. Semiconductor memory is the most rugged storage media, but its storage capacity is limited as compared to the other storage media, especially under a cost per MB analysis. Based upon existing technologies, the current cost to storage one MB of data in semiconductor RAM is currently around $60; on a magnetic disk, the cost decreases to about $1.60 per MB; and on an optical disk, the cost drops to only about 16 cents per MB. There are great cost savings to be had by successfully using optical disks as the preferred mass storage medium, provided that the environmental limitations can be overcome.
There are many computer systems in use today that are required to operate in severe environmental conditions. These conditions may be present in factories, at sites in potential earthquake areas, on ground transportation, on board ships, submarines, aircraft, an other platforms that presently utilize magnetic tape technology for their mass storage media. On known shipboard systems, magnetic tapes are used to record the data transfers between computers on a ship and between a computer on a ship and the outside environment. These magnetic tapes hold about 40 MB; and, in peak periods of activity, a magnetic tape is filled with data and a new magnetic tape must be mounted on the tape drive by a computer operator every five minutes. Many of these computer systems are being swamped by the larger amounts of data being reported by new sensor systems. There is a need to replace the old technology with the latest in high-speed, high-storage computer technology which is readily available in the commercial market. Commercial computer equipment is ordinarily not designed to withstand the stresses of severe environmental conditions.
In regard to the optical disk memory system, there are no known commercially available systems that provide significant improvements in the outer limits of successful operation in severe environmental conditions. Additionally, the anticipated cost of a ruggedized optical disk memory system and associated ruggedized optical disk cartridges is prohibitively expensive; proposals project prices an order of magnitude higher than current commercial prices. These proposed optical disk memory systems still do not fulfill the typical severe environmental system requirements. Thus, optical disk memory systems are not available for use on these types of computer systems.
What is needed is a mechanism by which the advantages of the huge storage capability of optical disks can be combined with the reliable operation of magnetic disks and semiconductor memory in a flexible, cost-effective manner. The solution to this problem will be applicable to a variety of situations where reliability, access time, storage capacity, and cost are critical factors in determining the success of a computer system operating in a severe environment.
In a traditional memory hierarchy, strategies have been developed to store information in a storage media based on frequency of use and speed of access. Thus, the most frequently used information would be stored in the shortest access time memory such as a semiconductor memory; frequently used information would be stored on a magnetic disk; and infrequently used information would be stored with a relatively slow access time on an optical disk.
The memory allocation strategy is significantly different for a data recording system operating in a severe environment, such as a military mission, because of unique requirements. There the objective is to store all information on the optical disk in real-time. The optical disk is then removed when full or at the end of the mission for later analysis and/or archival purposes. The data may be required to be archived for at least 10 years. Optical disks are ideal for such mass storage and for fulfilling this long-term archival requirement. During the mission, in a computer system utilizing optical disks, magnetic disks, and semiconductor memory, information can be stored on any of these storage media, depending on the current environmental conditions. All information stored on the magnetic disk or the semiconductor memory must be merged into the information stored on the optical disk as time and conditions permit, thereby allowing a computer operator to remove the complete mission information stored on optical disk.